The present application is related to the field of signal amplification and more particularly to a system for predistorting multiples signals prior to amplification to achieve a more linear amplification.
In an ideal linear amplifier, the output power (VOUT2) is equal to the input power (VIN2) times a constant K that does not vary with the input power. Similarly, the input signal phase ("THgr"IN) is equal the output signal phase ("THgr"OUT). In an actual amplifier, however, both the output power and the output phase vary from the ideal output power and the ideal output phase. Typically, the variation from the ideal output power and phase is a function of input power. Referring to FIGS. 1 and 2 ,diagrams of output power and carrier phase rotation as a function of input power for a non-ideal (real) amplifier are depicted. In FIG. 1, the output power of an actual amplifier is diagramed as a function of input power. The response of an ideal amplifier is represented by the straight line 100 where the slope of line 100 is equal to the desired gain of the amplifier denoted by K. FIG. 1 further indicates a real response curve 102 representing the output power of a real amplifier as a function of input power.
Typically, real response curve 102 includes three sections as indicated by reference numerals 104, 106, and 108. A first region 104 ,referred to herein as linear region 104, typically includes portions of response curve 102 representing input powers in the vicinity of zero. In first region 104, the response curve 102 closely tracks the ideal response curve 100. Thus, in linear region 104, the real amplifier represented by response curve 102 closely resembles an ideal amplifier. As the input power is increased, however, a real amplifier typically enters a second (compression) region 106 in which response curve 102 begins to roll off of ideal response curve 100. As the input power is further increased, the real amplifier represented by response curve 102 enters a third (saturation) region 108 in which the output power is essentially independent of input power as the real amplifier reaches a maximum obtainable output power.
Referring now to FIG. 2, response curve 201 represents the carrier phase rotation of a real amplifier as a function of input power. From inspection of response curve 201, the carrier phase rotation, which indicates the differential between the input signal and the output signal phase, is substantially equal to zero at low input powers indicating little or no phase shift. As the input power is increased, however, the carrier phase rotation increases in magnitude as indicated by the descending slope of response curve 201 .
Typically, it is highly desirable to eliminate the non-linearity of real amplifiers represented by response curves 102 and 201 in FIGS. 1 and 2 respectively. Therefore, it would be highly desirable to implement a circuit, method, and system to compensate for the non-linearity associated with real amplifiers such that the output of the amplifier would more closely resemble the output of an ideal amplifier. It would be further desirable if the implemented circuit, method, and system did not significantly increase the cost, complexity, and reliability of the amplification system. It would be still further desirable if the invention were able to accommodate multiple input signals.